1. Field of the Invention
This invention relates to systems for rotating the polarization of a linearly polarized beam, and more particularly to systems that employ a bidirectional interferometric coupler in the optical path leading to and from a frequency-shifting phase conjugate mirror (PCM) to produce a net polarization rotation.
2. Description of the Related Art
There are several applications in which the polarization angle of a linearly polarized beam needs to be rotated. Of particular concern are high power laser designs in which an input laser beam is amplified by a system that includes a PCM. Such systems are described in Rockwell, "A Review of Phase-Conjugate Solid-State Lasers", IEEE Journal of Quantum Electronics, Vol. 24, No. 6, June 1988, pages 1124-1140. In such systems the output beam is returned along the same path as the input, but in the opposite direction. The polarization of the output beam is rotated 90.degree. from the linearly polarized input beam, thereby allowing a polarizing beamsplitter to separate the low power input from the high power output beam.
A number of ways have been developed to produce a 90.degree. polarization rotation. One method uses a Faraday rotator, based upon the Faraday effect, in which certain materials act as polarization rotators when placed in a static magnetic field, with the sense of rotation governed by the magnetic field direction. The direction of rotation does not reverse when the beam direction is reversed, so that a beam that makes two passes through a Faraday rotator in opposite directions undergoes twice the rotation. Faraday rotators are described in Saleh et al., Fundamentals of Photonics, John Wiley & Sons, Inc., 1991, pages 223-233.
Although Faraday rotators achieve acceptable and reliable performance, they are relatively expensive, and large aperture devices are quite large and heavy because of the requirement for a uniform magnetic field to induce the polarization rotation. This is particularly troublesome for applications such as laser cutting and welding devices, which employ a focusing head that has relatively small weight limitations. These weight limits are difficult to meet with a Faraday rotator.
Another approach to polarization rotation uses an interferometric output coupler (IOC) that couples a beam into and out of a multi-pass phase conjugate amplifier chain. It exploits the fact that a PCM that is based upon Brillouin scattering imposes a frequency shift upon its output beam; this frequency shift is used by the IOC to produce a 90.degree. polarization rotation. The input beam passes through the interferometer, which alters its polarization, on the way to the PCM. After processing by the PCM, the return beam is directed back in the opposite direction through the interferometer, and undergoes a polarization alternation in the opposite sense to its first pass. However, since its frequency has been shifted, the return beam's polarization is altered by a different amount than the input beam's. The system is set up so that this difference translates to a 90.degree. polarization rotation.
The basic operation of an interferometer of this type is illustrated in FIG. 1. It includes a pair of polarizing beamsplitters PBS1 and PBS2, a pair of folding mirrors M1 and M2, and a porro prism 2. The input beam 4 is assumed to be linearly polarized at an angle of 45.degree. relative to the plane of the drawing. This is illustrated by the polarization vectors A, B and C, which are taken looking along the axis of input beam 4. Vector A represents the beam's polarization at a particular instant in time, and includes equal components Ay and Ax in the vertical and horizontal directions, respectively. (The terms "vertical" and "horizontal" are arbitrary, and are used herein only for ease of explanation to illustrate the division of a polarization vector into mutually orthogonal components.) Vector B represents the polarization one-half period later, at the input beam frequency, when it is directed 180.degree. to the initial vector A. Vector B consists of equal vertical and horizontal components By and Bx, which are reversed 180.degree. from Ay and Ax, respectively. The polarization components in the vertical and horizontal directions are in phase with each other, reaching their maximum positive and maximum negative extents simultaneously with each other. The resultant linear polarization over time is illustrated by vector C.
The input beam 4 is transmitted through a polarizing beamsplitter PBS3, which is oriented to transmit the input beam but to reflect a beam whose polarization is rotated 90.degree. with respect to the input beam. The input beam's polarization is not changed by the beamsplitter PBS3, and includes equal and in-phase vertical and into-the-page components y and x.
The polarizing beamsplitter PBS1 splits the input beam into two equal, orthogonally polarized subbeams 6 and 8 that respectively propagate towards the prism 2 and the mirror M2. The subbeam 6 retains the into-the-page polarization x of the original input beam, while subbeam 8 retains the vertical polarization y. Subbeam 6 is reflected back, parallel to but offset from its original path, by the prism 2, and is then reflected by mirror M1 onto the second polarizing beamsplitter PBS2. The second subbeam 8 propagates directly to mirror M2, from which it is also reflected onto PBS2.
The various system elements are oriented so that the two subbeams 6 and 8 are recombined by PBS2 into a single net beam 10, which is directed into a PCM 12. However, it can be seen that subbeam 6 has traveled a longer path between PBS1 and PBS2 than has subbeam 8. Specifically, the extension of the path for subbeam 6 to and from the prism 2 results in this travel differential (there will normally be some differential even without the addition of the extra path length due to prism 2). In general, this travel differential results in the x and y polarization components of the recombined beam 10 being out-of-phase with each other. This is illustrated by the polarization vectors D, E and F associated with the recombined beam 10; these vectors are again taken looking along the beam axis. At the particular instant when vector D occurs, its vertical component Dy is at its maximum positive extent, while its horizontal component Dx is illustrated as being at only a portion of its maximum positive extent. The situation one-half period later is illustrated by vector E, whose vertical component Ey is at its maximum negative extent, but whose horizontal component Ex is at only a portion of its maximum negative extent. The resulting polarization state F of the recombined beam is generally elliptical over time.
The recombined beam 10 is reflected by the stimulated Brillouin scattering PCM 12 and returns to PBS2. From there it is divided into orthogonally polarized components that travel through the interferometer in a reverse pass from the original subbeams 6 and 8, and are recombined as an output beam at PBS1. The optical path lengths of the two subbeams are made unequal by an amount .DELTA.L. This differential is set by an appropriate positioning of prism 2 such that .DELTA.k.DELTA.L=.pi., where .DELTA.k is the wavevector difference arising from the frequency difference between the input and output beams. This conditions ensures that, after the return beam completes its pass through the interferometer, the final output beam polarization is orthogonal to the input polarization. The output beam 14 is then deflected by PBS3 to separate it from the input beam.
The output beam polarization is illustrated by polarization vectors G, H and I. Since the polarization has been rotated 90.degree. from the input beam, the vertical and horizontal components Gy and Gx of vector G are 180.degree. out-of-phase with each other, with the vertical component Gy reaching its maximum positive extent at the same time the horizontal component Gx reaches its maximum negative extent. One-half period later (at the new frequency imposed by the phase conjugator), the polarization vector H has vertical and horizontal components Hy and Hx that are respectively at their maximum negative and positive extents, 180.degree. from their orientations for vector G. The result is an output linear polarization I that is rotated 90.degree. with respect to the input linear polarization C.
The various elements of the FIG. 1 system have been described in a number of publications, specifically, Basov et al., "Laser interferometer with wavefront-reversing mirrors", Sov. Phys. JTEP, Vol. 52, No. 5, November 1980, pages 847-851; Andreev et al., "Locked Phase Conjugation for Two-Beam Coupling of Pulse Repetition Rate Solid-State Lasers", IEEE J. of Quantum Electronics, Vol. 27, No. 1, January 1991, pages 135-141; Andreev et al., "Applications of Brillouin Cells to High Repetition Rate Solid-State Lasers", IEEE J. of Quantum Electronics, Vol 28, No. 1, January 1992, pages 330-341; and Andreev et al., "Single-mode YAG:Nd laser with a stimulated Brillouin scattering mirror and conversion of radiation to the second and fourth harmonics", Sov. J. Quantum Electronics, Vol. 21, No. 10, October 1991, pages 1045-1051.
When implemented in practice, the IOC concept of FIG. 1 suffers from the fact that it involves five separate optical components. These components must be precisely aligned to ensure that the two output beam components are parallel and perfectly overlapping spatially as they leave PBS2, after the first pass through the interferometer. Alternate configurations are described within the articles referenced above that reduce the number of components to three, but they still require sensitive alignments relative to one another. The adjustment mechanisms that are necessary to precisely align the various components work against the desired reduction in weight, size and complexity.